1. Field of the Invention
The present invention relates to a radiation curable composition and a process for producing thin, solid, polymeric films, by liquid deposition on a substrate with subsequent ultraviolet radiation, plasma radiation and/or electron beam (e-beam) radiation curing. Each of the liquid deposition, e-beam curing, plasma curing, and UV curing are done in a vacuum chamber. The radiation curable composition comprises components which do not go into a gas phase or vapor phase under the vacuum conditions. The composition has a first component which is polymerizable or crosslinkable in the presence of an acid; and a cationic photoinitiator which generates an acid upon exposure to ultraviolet radiation, plasma radiation, electron beam radiation or combinations thereof, thus causing polymerizing or crosslinking of the first component. The UV radiation and plasma radiation are generated in-situ by irradiating a gas within the vacuum chamber which generates UV radiation and/or plasma radiation upon exposure to electron beam radiation.
2. Description of the Related Art
There is great commercial interest in applying protective and/or functional coatings over metalized film substrates directly inside of a vacuum chamber and curing them via electron beam, UV and plasma irradiation. A benefit of such curable compositions is that they are essentially completely solid when cured and do not transfer into the gas or vapor phase under the vacuum. Applying solid curable coatings under vacuum is beneficial for coating uniformity and adhesion to non-oxidized metal surfaces. This is beneficial in comparison to applying electron beam curable coatings in air over oxidized metal surfaces.
Thin metallic and polymeric films add or promote desirable properties for particular applications. For example, foils used to preserve food need to have very low permeability to oxygen; the exterior surface of packaging material has to be capable of accepting printing inks; and packaging materials for electronic products also require a limited amount of conductivity to dissipate electrostatic charges. It is desirable and sometimes necessary to modify the physical properties of polymeric films to improve their suitability for the intended purpose. Preferably, the films are directly formed with a composition and molecular structure characterized by the desired properties. Thin films of metals and polymers are formed by deposition onto appropriate substrates by a variety of known processes, most notably through film formation by wet chemistry or vapor deposition. Chemical processes produce soluble thermoplastic as well as insoluble thermoset polymers and involve the use of solvents; thus, film formation is achieved through solvent diffusion and evaporation. As a result, these processes require relatively long residence times and the undesirable step of handling solvents.
Vapor deposition processes involve the evaporation of a liquid monomer in a vacuum chamber, its deposition onto a cold substrate, and subsequent polymerization by exposure to electron beam or ultraviolet radiation. U.S. Pat. Nos. 6,270,841 and 6,447,553 illustrate a liquid monomer from a supply reservoir which is atomized in a heated evaporator section of a vacuum deposition chamber where it flash vaporizes under vacuum. The resulting monomer vapor passes into a condensation section of the unit where it is vapor applied onto a substrate, condenses and forms a thin liquid film upon contact with the cold surface of the substrate. The liquid deposited film is then cured by exposure to an electron beam or ultraviolet radiation source. A problem with such a technique is that the vaporized composition coats much of the inside of the equipment inside the vacuum chamber, and then cures into an unwanted solid on the equipment when irradiated. Such unwanted solids are difficult to remove.
Traditionally, electron beam curable coatings are mixtures of (meth)acrylate functional pre-polymers, oligomers and monomers that can undergo free-radical polymerization under exposure to electron beam irradiation. Typically, electron beam free radical polymerization is inhibited by the presence of oxygen and therefore electron beam coatings must cure under a nitrogen blanket. The complete curing requires a substantial electron beam dose.
According to this invention, a radiation curable composition is formed comprising a first component which is polymerizable or crosslinkable in the presence of a sufficient amount of an acid; and a cationic photoinitiator which generates a sufficient amount of an acid upon exposure to sufficient ultraviolet radiation, electron beam radiation, plasma radiation or combinations of two or more of ultraviolet radiation, plasma radiation and electron beam radiation, to cause polymerizing or crosslinking of the first component. The radiation curable composition is applied in liquid form onto a surface of a substrate under vacuum conditions in a vacuum chamber. The radiation curable composition does not substantially go into a gas phase or a vapor phase under the vacuum conditions. An important feature of the invention is introducing a gas into the chamber, which gas generates and emits ultraviolet radiation, plasma radiation, or combinations of ultraviolet radiation and plasma radiation upon exposure to electron beam radiation. The composition is further exposed to the gas generated ultraviolet radiation and/or plasma radiation, and optional e-beam radiation thus providing curing of the composition. The cationic photoinitiator generates an amount of an acid under the influence of the electron beam, plasma and/or ultraviolet radiation. The acid causes at least polymerizing or crosslinking of the first component.
UV cationic chemistry is well known for outstanding adhesion to plastic substrates and metals as shown in U.S. Pat. Nos. 6,284,816; 6,489,375; and 6,451,873. In most practical applications, cationic polymerization takes place under UV irradiation when cationic photoinitiators, such as onium salts, for example. sulfonium or iodonium hexafluoroantimonate or hexafluorophosphate disassociate, forming strong Lewis acids, capable of reacting with epoxy, vinyl ether or oxetane functional groups. It is also known that cationic polymerization can take place under e-beam irradiation as shown in U.S. Pat. Nos. 5,260,349 and 5,877,229. It is further known that e-beam cationic polymerization can take place inside of a vacuum chamber as in U.S. Pat. No. 6,468,595. E-beam cationic polymerization requires the presence of an onium salt photoinitiator. Unfortunately the rate of e-beam induced cationic reaction is relatively low in comparison with UV induced polymerization. This limits use of e-beam cationic polymerization in high speed coating applications taking place in a vacuum metallization chamber.
According to the present invention, introducing a flow of various gases or blends of gases through the electron generated electrodes inside of a vacuum chamber leads to the emission of light containing UV spectral output and/or plasma electrons, that is useful for polymerization. For further enhancing of the rate of electron beam, plasma radiation and ultraviolet light radiation induced polymerization, a photosensitizer, such as anthracene, isopropylthioxanthone or phenothiazine, which is capable of transferring energy from the visible and high ultra-violet ranges of light spectra down to lower wavelength ultra-violet ranges, may be included.